Abstract: With increasing in global air temperatures as well as increased urbanization, cities are needing to incorporate innovative urban heat mitigation into their strategic sustainability plans. Urban tree canopies help mitigate urban heat through evaporative cooling and shading and are valued for providing numerous ecosystem services. In Boulder, Colorado, ash (Fraxinus sp.) trees make up around 20% of Boulder’s urban forest. Emerald ash borer (Agrilus planipennis) has been detected and puts over 20% of the urban canopy at a high likelihood of beetle infestation and mortality. The loss of these mature ash trees will significantly reduce urban shade, and we hypothesize there may be a detectable increase in local air temperature after ash tree removal. The effects of ash removal on local microclimates have not been studied in Colorado and this summer 2018 study is expected to give insight to the immediate changes in local weather that residents and wildlife may experience with significant urban forest canopy loss. We surveyed air temperature in an exploratory pilot study in Boulder, Colorado in 2018 to look at the immediate impact of ash removal on air temperature. Findings show

With a larger study, we may be able to provide predictions for local urban heat dynamics under change tree canopy, as well as baseline values for restoration and intermediate intervention plans for shade.

Research question: How is ambient air temperature immediately impacted by green ash tree (Fraxinus americana) removal?

Hypothesis: Air temperature will increase at the location of the removed ash trees (#100e and 101w) due to increased solar radiation and decreasing shade and evapotranspiration.

Methods: Air temperature was measured in four locations along Boulder creek (Photo 1.) from September 17-24. Removal of the adjacent ash trees #100e and 101w occured on September 20th between 8am - 11am.

Photo 1. Aerial view of the study area, North of Boulder Creek, Boulder, Colorado.

Photo 1. Aerial view of the study area, North of Boulder Creek, Boulder, Colorado.

Photo 2. Photo of treatment site where two green ash trees (Fraxinus americana) infected with Emerald Ash Borer beetles to be removed Sept 24, 2018 from Boulder Creek, Boulder, Colorado. a) showstreatment tree 100e and b) shows treatment tree 101w.

Photo 2. Photo of treatment site where two green ash trees (Fraxinus americana) infected with Emerald Ash Borer beetles to be removed Sept 24, 2018 from Boulder Creek, Boulder, Colorado. a) showstreatment tree 100e and b) shows treatment tree 101w.

Photo 3. a) shows the control tree (Fraxinus americana) that was not removed during the study and b) shows the control site with no trees along Boulder Creek, Boulder, Colorado.

Photo 3. a) shows the control tree (Fraxinus americana) that was not removed during the study and b) shows the control site with no trees along Boulder Creek, Boulder, Colorado.

Figure 5. Temperature values from the four sensors under the two treatment ash trees that were removed (101w, 100e), the control ash tree (103), and the sun control with no tree canopy showing before tree removal (Sep 16-19, 2019) and after removal (Sep 21-24, 2019) .

Figure 5. Temperature values from the four sensors under the two treatment ash trees that were removed (101w, 100e), the control ash tree (103), and the sun control with no tree canopy showing before tree removal (Sep 16-19, 2019) and after removal (Sep 21-24, 2019) .

Looking at the frequency of all temperatures logged, we see some small differences, most notable being the maximum temperatures reached in the before removal period to be 90.9 degrees F in the sun treatment.

Figure 5. Temperature data from the four sensors under the two treatment ash trees that were removed (101w, 100e), the control ash tree (103), and the sun control with no tree canopy showing before tree removal, during removal, and after removal

Figure 5. Temperature data from the four sensors under the two treatment ash trees that were removed (101w, 100e), the control ash tree (103), and the sun control with no tree canopy showing before tree removal, during removal, and after removal

Here, we see that there may be some noticable variation between the two treatment trees between before and after removal, but it is difficult to see clear differences at this full data scale. We can see before ash removal that the sun sensor logged values that surpassed 90 degrees (max = 90.9 F), whereas neither the treatment nor control trees broke 85.5 degrees F.

Figure 6. Maximum temperature values for the two treatment ash trees that were removed (101w, 100e), the control ash tree (103), and the sun control with no tree canopy on September 17 at 1:22pm

Figure 6. Maximum temperature values for the two treatment ash trees that were removed (101w, 100e), the control ash tree (103), and the sun control with no tree canopy on September 17 at 1:22pm

By zooming into the maximum values logged during the before removal period on September 17 at 1:22pm, we see the sun sensor logged a maximum temperature of 90.9 F, and the treatment and control tree sensors were approximately 6-7 degrees F less. After removal, the sun sensor still logged the highest maximum temperature on September 22, 7:12am at 88.8 F, and the control 103 tree had the lowest temperatue at that time 84.5 F. The treatment trees, however were less than 1 degree higher than the control tree.

Figure 7. Minimum temperature values for the two treatment ash trees (101w, 100e), the control ash tree (103), and the sun control before removal on Sept 19 at 7:25am and after removal on on September 22 at 7:12am

Figure 7. Minimum temperature values for the two treatment ash trees (101w, 100e), the control ash tree (103), and the sun control before removal on Sept 19 at 7:25am and after removal on on September 22 at 7:12am

By zooming into the minimum values logged during the after removal period on September 19 at 7:25am, we see the control tree sensor logged the lowest temperature at 54.39 degrees F, however the sun and treatment tree sensor were all 1-2 degrees higher than the control tree. After removal, the control tree sensor still logged the lowest minimum temperature on September 22, 7:12am at 43.58 F, with the sun and treatment trees within about 1 degree of the control tree.

Figure 3.

Figure 3.

If we look at the differences in temperature among each pair of sensors, we see that 100e and 101w treatment trees most often have a difference in temperature of 0 degrees (red line), which is to be expected because they are adjacent and receiving the same removal treatment. In both the before and after removal period, temperature is most often higher by about 0.5 degree in the sun control compared to the control tree (pink line). The treatment trees were most frequently 1-2 degrees cooler than the sun control before the removal, and 0-1 degrees cooler after removal (turquoise and blue lines). The difference among the control tree and treatment trees is unexpected, with the treatment trees being most frequently warmer by approximately 1 degree than the control tree before removal and less than a degree after removal, however a slight overal shift of the This suggests that temperature differences in ash removal may not strongly differ over the course of a day, but the extreme values early in the morning and midday may be the greatest source of differences in the course of tree removal.

Figure 2.

Figure 2.

In this figure, we can more clearly see see that the sun sensor was warmer than the control tree (pink boxes) before and after removal. The sun control was 1-2 degrees warmer on average than the treatment trees before the treatments, and after removal the sensors logged more similar temperatures with the sun control only 0-1 degrees warmer on average. There was a slight increase(<1 degree F) on average when comparing the before difference between the control trees and treatment trees to the after removal difference (yellow and green boxes).

Figure 4.

Figure 4.